Filter circuits, especially for battery charger circuits, are variously known and are used e.g. in traction vehicles for the railways. FIG. 6 shows a block diagram of a battery charger circuit 1 with a battery 2, which generates a nominal voltage of 110V and has an internal resistance R1. The battery 2 of the traction vehicle is charged through the charge regulator of the on-board supply system, which is represented in the block diagram by an ideal generator A and a diode D1, and charges the battery with a pulsating DC voltage (square-wave voltage) with a maximum value of 230V at 50 Hz.
The square-wave voltage U with a maximum value of 230V at point B of the battery charger circuit 1 is shown in FIG. 7a dependent on the time, the current I at point J of the battery charger circuit 1 in FIG. 7b. 
A filter circuit 3 according to prior art for the battery charger circuit 1 is likewise shown in FIG. 6. The filter circuit 3 contains a low-pass filter 5 with a first capacitor C1 with a capacitance of 4700 μF and a first resistance R3 of 2.5Ω. Pre-connected in series to the low-pass filter 5 is a second, identically constructed low-pass filter 4 with a second capacitor C2 and a second resistance R2, which have the same capacitance as the first capacitor C1 and the same resistance value as the first resistance R1 respectively. Electronic equipment linking to the output of the filter circuit 3 is represented in the block diagram of FIG. 6 by a load resistance RL1 corresponding to the input resistance of the electronic equipment. The filter circuit 3 is used for smoothing the voltage supplied by the battery charger circuit 1.
The measured output voltage U of the filter circuit 3 at point C is shown in FIG. 7d, the measured output current I at point K in FIG. 7c for a load resistance RL1 of 28Ω. The maximum value of the output voltage at point C has a value of 155V for a maximum current of 5.53 A. For the electronic equipment following the filter circuit 3, the output voltage of the filter circuit 3 is the relevant value, since in the present case the electronic equipment tolerates a maximum voltage of 155V (broken line in FIG. 7d). A further increase in the output voltage of the filter circuit 3 should therefore be avoided even for a heavier load on the filter circuit 3, determined by a greater input resistance of the electronic equipment.
The output voltage of the filter circuit 3 at point C for a load resistance RL1 raised to 28 kΩ (in comparison to FIG. 7, increased by a factor of 1000) is shown in FIG. 8d, the output current at point K in FIG. 8c. The maximum output voltage at point C is 186V, the maximum current at point K is only 6.65 mA, for which reason it is represented in FIG. 8c based on the chosen scale as a horizontal line at the zero line. The amperage at point J shown in FIG. 8b reduces in comparison to the maximum amperage of 19.1 A of FIG. 7b to a maximum 12.5 A, while the maximum voltage at point B in FIG. 7a as well as FIG. 8a is at a maximum 230V.
It is problematic here for the electronic equipment following the filter circuit 3, that its maximum tolerated input voltage of 155V for a load resistance RL1 (input resistance) of 28 kΩ is exceeded with the output voltage of 186V (FIG. 8d). With the filter circuit 3 it is not possible to convert the widely fluctuating input voltage from the battery charger circuit 1, into a stabilized output voltage.